Crohn's disease and ulcerative colitis (inflammatory bowel disease, IBD) are important clinical problems, but molecular targets for therapeutic immune intervention remain elusive. Intestinal dendritic cells (DCs) and macrophages (M?s) play a pivotal role in mediating mucosal tolerance and suppressing inflammation. In IBD, these cells lose their tolerogenic properties resulting in uncontrolled intestinal inflammation. However, the molecular pathways that program these cells to a tolerogenic state rather than to an inflammatory state are not known. We have identified a new and previously unsuspected role for the -catenin signaling pathway as a key molecular regulator of tolerogenic phenotype in intestinal DCs and M?s. -catenin is downstream of three sets of ligands widely expressed in the gut (TLR ligands, wnt ligands and E-cadherin), and ablation of -catenin in these cells causes loss of tolerance. The current proposal will focus on the mechanistic role of the -catenin pathway in regulating key downstream effector mechanisms, and test its relevance in in vivo models of colitis and oral tolerance. Specific aims in the current proposal are (i) to understand the molecular mechanisms by which -catenin/TCF pathway regulates the expression of three key immune regulatory genes - IL-10, RALDH and IDO - in intestinal DCs and M?s (Aim 1), (ii) to understand the functional and biological role of this pathway in intestina DCs and M?s in T regulatory cell differentiation and expansion (Aim 2), and (iii) their ability to limit intestinal inflammation and promote oral tolerance (Aim 3). The successful completion of the proposed studies will provide new mechanistic insights into how the -catenin/TCF pathway in intestinal DCs and M?s regulates a balance between tolerance and inflammatory responses, and will provide a mechanistic rationale for targeting this pathway in IBD. Pharmacological activators of -catenin pathway already exist, and more are in development and the proposed studies will provide a rationale for the development of an entirely new class of agents that may have significant therapeutic impact in treating IBD. PUBLIC HEALTH RELEVANCE: A fundamental puzzle in mucosal immunology is how the immune system decides between tolerogenic response versus inflammatory response against commensals, pathogens and harmless food antigens. Intestinal dendritic cells (DCs) and macrophages (M?s) play a central role in this complex decision making process, but the molecular pathways that program these cells are not known. We have identified a novel molecular pathway that programs intestinal dendritic and M?s to a tolerogenic phenotype and limits intestinal inflammation. The information gained from these studies will provide will provide a rationale for the development of an entierly new class of agents that may have significant therapeutic impact in treating inflammatory bowel disease.